The student is studying the movement of substances across the cell membrane. She understands that some substances combine with special transport proteins to be carried across the cell membrane. The substances can move only from an area of higher concentration to one of lower concentration. This process is known as:

Facilitated diffusion

Explanation:

Facilitated diffusion is the transport of hydrophilic molecules that can not freely cross the membrane. Channel protein and many carrier proteins are in charge of this transport. When uncharged molecules cross the membrane, they do it according to their concentration gradients, going from the more concentrated side to the lower concentrated one. When ions need to cross the membrane, the process depends on an electrochemical gradient.  

Facilitated diffusion is a passive transport process because they only depend on electrochemical gradients, so they do not need any energy to occur.

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